1. Field of the Invention
The present invention relates to a method for producing a steel material having a high fatigue strength and can be suitably used in power transmission in automobiles and industrial machines.
2. Description of the Related Art
In order to improve fatigue strength of a material such as marageing steel, solution heat treatment, aging treatment, and nitriding treatment are generally applied. A method for imparting further higher fatigue strength is disclosed in Japanese Patent Application Laid-Open (JP-A) No. HEI 2-154834. According to this method, after surface hardening treatment such as the nitriding, a shot-peening treatment is applied on the surface, and thereby compressive residual stress is imparted, resulting in a steel material having high fatigue strength.
However, in the prior arts such as the method set forth in JP-A No. 2-154834, there are problems as follows.
1. Since it is difficult to spray hard particles uniformly on front and back surfaces, residual stress varies, resulting in inability to obtaining predetermined fatigue strength.
2. In order to make the residual stress uniform, the hard particles have to be uniformly sprayed while changing spraying position, resulting in a longer operation time.
3. Since irregularities are formed on the surface owing to the spraying of the hard particles, it is difficult to control surface roughness and surface properties (mirrored surface, buffer mark, twill line, etc.) with an intention of applying, for instance, a lubricant and so on.